In another bloody weekend in Afghanistan, four American soldiers lost their lives when an Afghan police officer turned his gun on U.S. troops in Zabul. Later that day, an Afghan soldier opened fire on a vehicle he believed was carrying NATO soldiers. One foreign civilian was wounded in that attack, the Associated Press reports.

This is hardly the first time an Afghan ally has gone turncoat.

CNN's Anna Coren met with an Afghan man who claims to have killed two of his American trainers in Wardak province in October 2009. His face covered in a shawl to remain anonymous, the 30-year-old man explained how he waited for the right moment and decided to attack when the American troops had entered a school and taken off their body armor to rest.

Explaining his decision to turn on his trainers, the Afghan told CNN that the Americans were "oppressing people" in Afghanistan. "They were burning copies of the Holy Quran and disrespecting it," the alleged attacker added.

The growing number of "insider attacks" has sparked concern among American lawmakers and raised fears that the assaults may pose significant challenges to the Obama administration's exit strategy in Afghanistan.

This weekend's assaults drew strong condemnation from Army Gen. Martin Dempsey, the U.S.'s top commander. Dempsey called the green-on-blue attacks "a very serious threat" to the war effort, according to the Associated Press. U.S. Defense Secretary Leon Panetta described the assaults as a "last gasp" of a Taliban insurgency, the AP added.

Afghanistan War, By The Numbers

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Start of War: Oct. 7, 2001

American soldiers hide behind a barricade during an explosion, prior to fighting with Taliban forces November 26, 2001 at the fortress near Mazar-e-Sharif, northern Afghanistan. (Photo by Oleg Nikishin/Getty Images)